Mobile genetic elements have profoundly influenced the evolution of bacterial pathogens. Many determinants of virulence and antibiotic resistance are borne by mobile elements, such as plasmids and bacteriophages, which are capable of horizontal transmission. During this grant cycle, we studied several aspects of the life cycle of CTXF, a filamentous phage that encodes cholera toxin, the principal virulence factor of the cholera pathogen Vibrio cholerae. We described novel mechanisms that mediate the integration of CTXF into the V. cholerae chromosome and that govern CTXF gene expression. We also expanded the scope of our work to explore several aspects of the molecular biology of SXT, a V. cholerae-derived integrating conjugative element (ICE) that encodes multiple antibiotic resistance genes. In recent years, SXT and closely related elements have become highly prevalent in V. cholerae clinical isolates. Our long-term goal is to decipher the molecular mechanisms that enable the dissemination of CTXF and SXT. We propose to explore 5 aspects of the molecular biology of CTXF and SXT that govern their transmission and evolution. Experiments in Aim I to elucidate mechanisms that control expression of RstR, the CTXF repressor, will reveal how two host factors, LexA and RpoS, contribute to the novel genetic circuitry governing CTXF production. In Aims II-V, we will explore the molecular bases for aspects of the SXT life-cycle that distinguish this mobile element from phages and plasmids. Studying these novel features of SXT will enhance our understanding of ICEs, an extensive and medically important set of mobile elements that have not been subject to intensive study, and will shed light on processes that have contributed to the prevalence and diversity of SXT-related ICEs in the gamma proteobacteria. In particular, we will explore II) steps in SXT conjugation that limit its transfer to new hosts, III) genes required for SXT maintenance in donor cells, IV) mechanisms for preventing redundant transmission of SXT to hosts in which it is already present and V) processes by which new/hybrid SXT-related ICEs are generated. Completion of these studies will enhance our understanding of the mechanisms that control of horizontal gene transfer and yield knowledge of fundamental aspects of prokaryote cell biology and pathogen evolution. Furthermore, these studies may provide valuable information for the creation of new antimicrobial agents and vaccines.